Field of the Invention
The invention relates to a meterable screen or halftone roller, especially for an anilox inking unit in a rotary printing machine, with a multiplicity of printing ink-accepting depressions disposed on the peripheral surface of the screen or halftone roller.
In the arts of gravure printing and offset printing, inking units are provided wherein printing ink is introduced via a doctor chamber into dimples or groove-like depressions formed in a screen or halftone roller, for example, an anilox roller, which subsequently transfers the printing ink, generally via one or more intermediate rollers, to the surface of a printing-form cylinder.
Screen rollers having depressions formed in the surface thereof by engraving, etching or with the aid of lasers have become known heretofore, the rollers being made, for example, of steel, chromium, copper and/or ceramic. A factor that is common to all the screen rollers is that they have dimples or groove-like depressions, the volume of which is defined during the production of the roller and, except for wear caused by the doctoring operation, can generally no longer be altered thereafter.
Because the quantity of ink transferred to the printing form or plate is determined by the volume of the dimples or the groove-like depressions formed in the screen or halftone rollers, and the quantity of ink required overall depends upon the size of the area of the printing form or plate that is to be inked, it is desirable to be able to alter the quantity of ink from the viewpoint of an optimum adaptation or matching of the quantity of ink supplied to the respective print job.
In this connection, in the case of rollers made of ceramic partly filled with copper, it has become known heretofore to remove copper from the filler by etching the roller, in order thereby to increase the volume of the dimples or grooves. In the case of these rollers, however, a reduction in volume can be produced only by grinding over the latter, however, this entails a comparatively great mechanical effort and, in addition, results in a reduction in the circumference of the roller, that has a detrimental effect upon the printing process.
It has furthermore become known from German Patent 44 08 615 to introduce into the dimples or depressions of a screen or halftone roller a piezoelectric material that expands or contracts when an electrical voltage is applied and, consequently, produces an alteration in the volume of the dimples or depressions. A disadvantage with respect to the aforedescribed screen or halftone roller is that, in order to effect an alteration in the volume of the dimples or depressions that is visible in the printed image, the piezoelectric material in each of the dimples or depressions has to have a very high voltage applied thereto, which results not only in the occurrence of technical problems during the production of the screen or halftone roller, but also, because of the risk of voltage flashovers, constitutes a considerable safety risk during the operation of the roller.